4chan isn't breaking any US laws

They break US laws every single day. Every loli in /b/ and /gif/ thread violate several laws and yes people do debate this endlessly which I will not, discuss that with lawyers that deal with CSAM. On that alone they could easily seize all the servers if they wanted to but that will never happen because like I said it's an goldmine for people self reporting they are going to shoot up a place or show intent for a myriad of other crimes. The feds would never throw away such an easy mode treasure trove nor would I expect them to. The site started glowing hard in 2008 and glowed even harder after 2012. I even showed people how to extract IP addresses using the hashes in the thread and post ID prior to their moving to Cloudflare and the users still went into full cope.

All of this aside it would be trivial to add the RTA header to the entire site. They could add it in the Cloudflare interface in a few seconds. It would cost them nothing. Only groomers would have their jimmies rustled even despite most of the groomers having moved to Roblox.